1. Field of the Invention (Technical Field)
The present invention relates to the reduction of intersymbol interference that occurs in closely-spaced data symbols in data transmission and data storage channels.
2. Background Art
One negative consequence of ever-increasing (linear) recording densities is the ubiquitous nonlinear distortion arising from closely-spaced recorded transitions. Among the dominant effects are partial erasure, for magneto-inductive heads, pulse asymmetry and saturation, for magneto-resistive heads, and transition shifting for both.
A number of methods for mitigating against nonlinear intersymbol interference (ISI) have been examined in recent years, including the random access memory-decision feedback equalizer (RAM-DFE), various RAM ISI-cancellers, various sequence detectors, neural network-based ISI-cancellers and Volterra equalizers. All of these techniques suffer from either high complexity or marginal effectiveness.
Communications through nonlinear ISI channels continues to be of great interest as available spectrum becomes scarce, pushing for greater bandwidth efficiency. Additionally, the need for power efficiency, such as for hand-held wireless devices or satellite transponders, often requires the use of power amplifiers running in the saturation region. Unfortunately, within such a bandlimited nonlinear environment, typical methods to increase data rate such as faster symbol rates or higher order modulation schemes suffer greatly. In many nonlinear channels, the distortion caused by the nonlinear element, such as a nonlinear power amplifier and bandlimiting filters, creates nonlinear ISI. Such distortion typically limits the system to low order constellations such as PSK. Higher order constellations are possible only if effective reduction of the nonlinear ISI is achieved.
Various proposed solutions have varying degrees of success. One method proposed adaptively predistorting the transmitted signal in such a way that the desired constellation is achieved after passing through the nonlinearity. While this is effective, it requires additional hardware, essentially a receiver, in the transmitter. Such additional hardware is a disadvantage for many power/size limited transmitters.
Other solutions focus on receive-side signal processing, and others have proposed the use of nonlinear equalizers in the form of Volterra filters. While these may be effective, the potentially large parameter space and sensitivity to noise remains an issue in some applications. The technique of the present invention is robust in the presence of channel noise, requires no additional transmit hardware, and has modest receiver hardware needs. The present invention builds upon work in the magnetic storage channel and digital communications fields to essentially extend the random access memory-decision feedback equalizer (RAM-DFE) idea to reduce xe2x80x9cprecursorxe2x80x9d nonlinear ISI components. The xe2x80x9cprecursorxe2x80x9d effects can be significant. Further, since a trellis detector or decoder is usually involved, the present invention incorporates the RAM into the trellis detector or decoder.
The present invention provides a unique approach to addressing the problem of ISI. A subset of RAM inputs is searched for the ISI yielding a minimum branch metric error. In the preferred embodiment, a RAM search method is used in cooperation with a Viterbi detector. The present invention is also compatible with Viterbi decoding, when the application is a communication channel with convolutional coding, and with BCJR-APP (acronym known to those of ordinary skill in the error-correcting codes art for xe2x80x9cBahl, Cocke, Jelinek and Ravivxe2x80x94A Posteriori Probabilitiesxe2x80x9d, as set forth in L. R. Bahl, et al., xe2x80x9cOptimal Decoding of Linear Codes for Minimizing Symbol Error Ratexe2x80x9d, IEEE Trans. Info. Theory, Vol IT-20, pp. 284-287, March 1974) detection and decoding when the application involves turbo codes.
Prior patents that disclose related technology include: U.S. Pat. No. 5,430,661, to Fisher et al., entitled xe2x80x9cAdaptive Decision Feedback Equalizer Apparatus for Processing Information Stored on Digital Storage Media;xe2x80x9d U.S. Pat. No. 5,132,988, to Fisher et al., entitled xe2x80x9cAdaptive Decision Feedback Equalizer Apparatus for Processing Information Stored on Digital Storage Media;xe2x80x9d U.S. Pat. No. 5,426,541, to Coker et al., entitled xe2x80x9cSelf-Equalization Method for Partial-Response Maximum-Likelihood Disk Drive Systems;xe2x80x9d U.S. Pat. No. 5,418,660, to Sato et al., entitled xe2x80x9cInformation Processing Apparatus for Processing Reproduction Signal Having Nonlinear Characteristics;xe2x80x9d U.S. Pat. No. 5,414,571, to Matsushige et al., entitled xe2x80x9cAdaptive Equalization Circuit for Magnetic Recording Apparatus Having High Error Immunity;xe2x80x9d U.S. Pat. No. 5,703,903, to Blanchard et al., entitled xe2x80x9cMethod and Apparatus for Adaptive Filtering in a High Interference Environment;xe2x80x9d U.S. Pat. No. 5,742,642, to Fertner, entitled xe2x80x9cSignal Processing Method and Apparatus for Reducing Equalizer Error;xe2x80x9d U.S. Pat. No. 4,852,090, to Borth, entitled xe2x80x9cTDMA Communications System with Adaptive Equalization;xe2x80x9d U.S. Pat. No. 5,107,378, to Cronch et al., entitled xe2x80x9cAdaptive Magnetic Recording and Readback System;xe2x80x9d and U.S. Pat. No. 5,166,914, to Shimada et al., entitled xe2x80x9cData Recording/Reproducing Apparatus with Two-Dimensional Equalizer for Crosstalk Cancellation.xe2x80x9d These references provide related technologies, however most do not utilize RAM-DFE techniques. The ""661 and ""988 patents to Fisher et al., disclose the use of a decision feedback equalizer; however, they introduce a design of a RAM-DFE that is capable only of mitigating the nonlinear ISI caused by post-cursor symbols. This would be effective when most of the nonlinear ISI is post-cursor, but when the precursor nonlinear ISI is significant, its effectiveness becomes marginal. The present invention introduces a solution to this problem by searching over all possible combinations of the precursor symbols while fixing the post-cursor symbols to past decisions. Then, the combination that gives the optimal branch metric is used, along with the post-cursor samples, to address the RAM. This method is termed RAM-search because it borrows the notion of employing a RAM from the RAM-DFE, and because it involves a search. The preferred embodiment of the present invention is termed the RAM-search-Viterbi-detector (RS-VD). The RS-VD uses a RAM-search method in cooperation with a Viterbi algorithm. This algorithm shows a considerable performance improvement in the bit error rate over other algorithms. This is due to the fact that post-cursor symbols are more reliable, and the error propagation is reduced. In summary, the present invention cancels both precursor and post-cursor linear and nonlinear ISI.
The present invention is a method of canceling intersymbol interference (ISI) for a signal bit ak at time k, where kxcex5{0, xc2x11, xc2x12 . . . } in data transmission and data storage channels, such as in magnetic recording, optical recording, image processing, image coding, satellite communications, wireline communications such as telephone lines, and wireless communications such as digital cellular telephones. The method is comprised of the steps of modelling the channel; training a RAM with known values for ak to acquire the model of the channel; searching for the optimal address for the RAM with past, present and future values of ak that will be used to cancel the ISI in the received signal; and canceling the ISI from the received signal. The training step comprises training with a training update algorithm. The modelling step can comprise modelling with a simple partial erasure model to produce a discrete time signal. The training step can comprise acquiring a model for a magneto-inductive head or a magneto-resistive head, or some other nonlinear channel model. The method can further comprise the step of equalizing the discrete time signal to a desired partial response target. Equalizing can be accomplished with either a long MMSE FIR filter equalizer or a zero-forcing equalizer. The partial response target can be a PR4 or an EPR4 if a partial response target is used. If a partial response target is used, the discrete time signal can be expanded into a Volterra series. When modelling the magneto-resistive head, it is preferably modelled as a memoryless device with a transfer function. Modelling with a transfer function can be characterized by a third-order polynomial. When modelling with a transfer function, asymmetry and saturation effects can be incorporated into the transfer function to produce the discrete time signal. Preferably, the discrete time signal is then filtered with a matched filter.
As with the magneto-inductive head, the magneto-resistive head""s discrete time signal can be equalized with an MMSE equalizer. In the method for canceling intersymbol interference, searching preferably comprises searching for the optimal bit combination in the feedforward path of ak that when used to address the RAM, minimizes the ISI for ak.
In the preferred embodiment for canceling intersymbol interference, the searching step is done in cooperation with decision-making on the data symbol values. The decision-making is accomplished with a detector which can be either a Viterbi detector, a Viterbi decoder, a BCJR-APP detector, or a BCJR-APP decoder. Preferably, each trellis state of the detector detection algorithm is equipped with the RAM trained with the channel model. Searching is then accomplished by deriving the address for the RAM from the paths into the trellis state of the detection algorithm, together with future ak bits. To derive the address, the branch metric of the detection algorithm is modified according to the minimum squared error of the ISI for ak.
The present invention is also for an apparatus for canceling intersymbol interference for a signal bit ak at time k, where kxcex5{0, xc2x11, xc2x12, . . . } in data transmission and data storage channels. The apparatus is comprised of means for modelling the channel; a RAM trained with known values for ak to acquire the model of the channel; means for searching for the optimal address for the RAM with past, present and future values of ak to be used in minimizing ISI in the received signal; and means for canceling the ISI. Preferably, the means for searching further comprises a decision maker such as a Viterbi detector, Viterbi decoder, BCJR-APP detector, or BCJR-APP decoder. Means for modelling, searching, decision-making, and canceling include well-known devices in the art such as computer software, firmware on a computer board or digital signal processor, or in hardware such as an integrated circuit.
A primary object of the present invention is to circumvent error propagation that arises in making tentative decisions in ISI-cancellers used in data storage channels and in data transmission channels.
A primary advantage of the present invention is that improvements of about three decibels are possible.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.